Prior to entering law enforcement, Lupe Valdez was an officer in the United States Army. During her time in the Army, she attained the rank of Captain.

Her law enforcement career began as a jailer, first in a county jail and then a federal prison. She then moved on to investigative roles as an agent of the General Services Administration, the U.S. Department of Agriculture and, finally, the U.S. Customs Service where she was a leader in the federal Counter Smuggling Initiative. With the creation of the Department of Homeland Security in 2002, she was made a Senior Agent, serving in that role until her retirement in 2004. In January 2004, Lupe Valdez retired to run for the office of Dallas County Sheriff.

On January 2, 2004, Lupe Valdez announced her candidacy for the Democratic nomination for Dallas County Sheriff. During the primary election, she faced three opponents, and finished as the highest vote-getter with 13,867 votes. She subsequently won a run-off election against future Dallas County Judge Jim Foster. Valdez won 73% of the vote in the run-off.

As she entered the general campaign, Valdez was widely considered the underdog in her general election race against Republican Danny Chandler. Chandler, a 30-year veteran of the Sheriff's Department, had defeated incumbent Sheriff Jim Bowles in the Republican primary. Bowles, who was tainted by corruption allegations, had held the office for 20 years.

The general election saw Valdez beat Chandler by 51.3% to 48.7% - a margin of some 18,000 votes. The election, combined with the fact that Valdez is female, Hispanic and a lesbian, made national headlines and was even reported overseas.

As an openly gay candidate for public office, Valdez's campaign won the backing of the Gay & Lesbian Victory Fund. She was sworn in on January 1, 2005.

Upon taking office as Dallas County Sheriff, Valdez faced a department that was wracked by poor morale, tainted by allegations of corruption and marred by the fact that the Dallas County Jail had begun failing state and federal inspections prior to her election. The jail had failed inspections because of poor sanitation conditions which endanger prisoners, many of whom have not ultimately been found to be guilty of any crime and are merely being held pending being formally charged or, released; a failing smoke evacuation system, unacceptable medical care, and a lack of sufficient guards to meet the legally required guard-to-inmate ratio.

Although the Dallas County Jail had begun failing state and federal inspections prior to Valdez being elected to office, the jail continued to fail inspections every year thereafter until 2010, when the jail passed certification by the State of Texas for the first time since 2003.[1]

Valdez formally filed for re-election to a second term on December 3, 2007.[2] She faced criticism from her opponents[3] and the Dallas Morning News.[4]

Valdez won the 2008 primary, narrowly avoiding a runoff by winning 50.85% in a four-candidate field on March 4, 2008.[5]

On November 4, 2008, Lupe Valdez was re-elected Sheriff of Dallas County with 388,327 votes to Lowell Cannaday's 322,808 votes, a margin of roughly 65,500. Valdez received over 99,000 more votes than the "Straight Democratic" option. She won in precincts across Dallas County, including formerly Republican areas including Valley Ranch in Irving and Mesquite. Her opponent won most precincts in far North Dallas, Richardson, Coppell, and the southern part of Irving.[6] She began her second four-year term on January 1, 2009.

In 2010, the Dallas County Jails passed inspection by the State of Texas for the first time since 2003. Completion of a new jail facility in 2009 and continued investment from Dallas County were cited as steps towards re-certification of the Dallas County jail system, which passed inspection once again in 2011.[7]

Also in 2010, Sheriff Valdez was elected to the Democratic National Committee[8] and was appointed by President Barack Obama to a committee regarding immigration reform.[9]

In early December 2011, Valdez filed to run for a third term as Sheriff.

Dallas County jails still failing state inspections,[17][18][19][20][21][22][23][24][25] though with improving levels of compliance at least partly due to Dallas County spending "millions" to improve sanitation and jail health.[26]

Taking 2 years to successfully pass her Texas Peace Officer Certification examination after being elected.[14]

Engaging in campaign activities while in uniform.[16][34] However, according to state law in Texas, it is not illegal for elected law enforcement officers, including sheriffs and constables, to campaign in their uniform.[citation needed]

Violating "long-standing policy" in Dallas County, which was upheld by a state district judge,[39] by authorizing the Discovery Channel to film a documentary without gaining required Commissioner Court permission.[26] This prompted the Commissioners Court to unanimously seek[40] (and ultimately get[41]) a restraining order against the Sheriff. The Dallas Morning News editorial board criticized the Sheriff's handling of the matter: "the positive publicity from finally meeting state standards would have been superior to that of a cable documentary and an unnecessary fight with those who hold her purse strings."[42] The legal costs to the county could be as high as $11,000.[39]

Average freeway wreck clearing time has increased from 16 minutes in 2004 to 45 minutes in 2007.[43]

Eliminating patrol officers and moving all of them to freeway patrols for a goal of "boosting ticket revenue".[43]